This application is based on French Patent Application No. 00 10 526 filed Aug. 10, 2000, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The present invention relates to an electrochemically active material consisting of a hydridable alloy that can be used in an alkaline electrolyte secondary storage cell negative electrode. It further relates to a method of fabricating the alloy. The invention also relates to an electrode including the active material and to a storage cell containing an electrode whose electrochemically active material is a hydridable alloy according to the invention.
2. Description of the Prior Art
Sealed nickel-metal hydride (Nixe2x80x94MH) storage cells are aqueous alkaline electrolyte secondary storage cells. The hydrogen reagent is stored in the mass of the hydridable alloy, which can absorb great quantities of hydrogen. The alloy must be able to store and return hydrogen, depending on whether the storage cell is being charged or discharged, at a sufficient speed under normal conditions of operation. It must also have an electrochemical capacity higher than that of cadmium, resist corrosion in potassium hydroxide, and not be toxic.
At present AB5 type hydridable alloys derived from LaNi5 are mostly used. The substance LaNi5 has a solid-gas reversible hydrogen absorption capacity equivalent to 370 mAh/g. However, its plateau pressure, which is of the order of 2 bar, is too high for use in a negative storage cell electrode, for which the plateau pressure must be from 0.01 bar to 1 bar. Also, this alloy has insufficient resistance to corrosion in concentrated potassium hydroxide.
To reduce the cost of LaNi5 type alloys, the lanthanum La is generally replaced with a mischmetal Mm whose typical composition is: La0.25 to 0.35 Ce0.45 to 0.55 Nd0.10 to 0.20 Pr0.03 to 0.07. For the same reason, attempts are made to reduce the proportion of certain costly metals, in particular cobalt, in the alloy. For example, U.S. Pat. No. 5,512,385 proposes a hydridable alloy containing little cobalt. The alloy comprises at least two phases and is represented by the general formula Mm NixMy, in which 5.0xe2x89xa6x+yxe2x89xa65.5 and M is at least one element from Al, Mn, Co, Cu, Fe, Cr, Zr, Ti and V. The alloy has the formula MmNiaAlbMncCudCoeFefCrg, for example.
It is generally accepted that the service life of a nickel-metal hydride storage cell is limited by corrosion of the hydridable alloy during cycling. To reduce corrosion, U.S. Pat. No. 4,487,817 proposes an alloy with the formula A Bm Cn in which 4.8xe2x89xa6n+mxe2x89xa65.4. In particular, A can be a mischmetal containing more than 50 wt % of Ce, approximately 25 wt % La and approximately 25 wt % of a mixture of other rare earths. B is at least two elements from Ni, Co, Cu, Fe and Mn, and C is at least one element in a particular atomic proportion chosen from Al (0.05-0.6), Cr (0.05-0.5) and Si (0.05-0.5).
U.S. Pat. No. 5,753,386 proposes an alloy having a long service life in cycling in a wide temperature range. The alloy is represented by the general formula: A NiaMbMxe2x80x2cTd where A is a mischmetal comprising La (50-70 wt %), Ce (1-30 wt %), Pr (0-10 wt %), Nd (0-10 wt %) and Y (0-10 wt %). M is at least one element from Co, Fe and Cu, Mxe2x80x2 is Mn and/or Al, and T is at least one element from B, Si, S, Cr, Ga, Ge, Mo, Ru, Rh, Pd, Ag, In, Sn, Sb, Bi, P, V, Nb, Ta and W. In the above formula, the atomic composition is as follows: 3.2xe2x89xa6axe2x89xa64.0; 0.4xe2x89xa6bxe2x89xa61.5; 0.3xe2x89xa6cxe2x89xa60.9; 0xe2x89xa6dxe2x89xa60.2 and 4.5xe2x89xa6a+b+c+dxe2x89xa65.6. The alloy is fabricated by melting a mixture of the consituents in the above proportions and then cooled rapidly, at a rate of at least 100xc2x0 C./s, and finally heat treated, preferably at a temperature from 500xc2x0 C. to 900xc2x0 C. Initially the additional constituents are uniformly dispersed in the alloy and a fine-grain intergranular phase is precipitated. If the sum (a+b+c+d) is outside the range 4.5-5.6 the quantity of the second phase is increased.
However, some applications require the storage cell to be able to resist high temperatures during long idle periods or periods of permanent overcharging between charging and discharging phases. In this case, the service life of the storage cell will depend on the ability of the hydridable alloy to resist corrosion in storage in a high-concentration aqueous alkaline electrolyte.
The object of the present invention is to propose an electrochemically active material in the form of a hydridable alloy having high resistance to corrosion during cycling and whose corrosion during storage is reduced compared to prior art materials.
The invention provides an electrochemically active material consisting of a CaCu5 structure single-phase hydridable alloy with the formula
MmNiaMnbAlcCodCre
in which Mm is a mischmetal containing at least 50 wt % La, where:
5.10xe2x89xa6(a+b+c+d+e), dxe2x89xa60.55, and 0.03xe2x89xa6exe2x89xa60.1.
For cost reasons, the atomic proportion of Co must be at most equal to 0.55, i.e. contain an amount of Co less than or equal to 7.5 wt % relative to the alloy. This reduces the resistance of the alloy to decrepitation. Increasing the stoichiometry beyond 5.10 remedies this drawback, but the plateau pressure then increases in an unacceptable proportion for use of the alloy as an electrochemically active material in an electrode. The aforementioned drawbacks are solved by using a mischmetal containing at least 50% La combined with an atomic proportion of Cr such that 0.03xe2x89xa6exe2x89xa60.1
The invention further provides an electrochemically active material consisting of a CaCu5 structure single-phase hydridable alloy with the formula
MmNiaMnbAlcCodCre
in which Mm is a mischmetal containing from 50 wt % to 80 wt % La, where:
3.7xe2x89xa6axe2x89xa64.3
0.3xe2x89xa6bxe2x89xa60.7
0.1xe2x89xa6cxe2x89xa60.4
dxe2x89xa60.55
0.03xe2x89xa6exe2x89xa60.1
5.10xe2x89xa6(a+b+c+d+e)xe2x89xa65.40
For values of (a+b+c+d+e) greater than 5.40 the capacity is reduced, the plateau pressure is increased and secondary phases are precipitated that are depleted or free of mischmetal in the CaCu5 structure matrix. The alloy is no longer a single-phase material. To retain sufficient resistance to corrosion during cycling, the following condition preferably applies: 5.15xe2x89xa6(a+b+c+d+e)xe2x89xa65.35.
In accordance with the present invention, Mm is a mischmetal consisting of a mixture of La, Ce, Nd and Pr. It contains from 50 wt % to 80 wt % La, preferably 50 wt % to 70 wt % and even more preferably 60 wt %. The amount of Ce in the mischmetal must not be less than 10 wt % to preserve sufficient resistance to decrepitation.
The atomic proportion of Mn is such that 0.3xe2x89xa6bxe2x89xa60.7 and preferably 0.45xe2x89xa6bxe2x89xa60.65.
The atomic proportion of Al must be greater than or equal to 0.1. With less Al the corrosion resistance of the alloy is no longer sufficient. The atomic proportion of Al is 0.1xe2x89xa6cxe2x89xa60.4 and preferably 0.15xe2x89xa6cxe2x89xa60.25.
In the alloy according to the invention, the atomic proportion of Co is 0.25xe2x89xa6dxe2x89xa60.55, preferably 0.25xe2x89xa6dxe2x89xa60.51, to achieve an acceptable cost, and even more preferably 0.35xe2x89xa6dxe2x89xa60.45.
During previous experiments on this family of alloys, it was observed that an atomic proportion of Cr greater than 0.1 atom of Cr for one atom of Mm led to low capacity per unit mass and slow activation. The atomic proportion of Cr is such that 0.03xe2x89xa6exe2x89xa60.1 and preferably such that 0.03xe2x89xa6exe2x89xa60.07.
The method of fabricating an electrochemically active material in accordance with the present invention comprises the following steps:
simultaneously melting the constituents of the hydridable alloy in an oxygen-free atmosphere,
rapidly cooling the molten mixture at a cooling rate of at least 10xc2x0 C.sxe2x88x921; and
annealing the alloy in an oxygen-free atmosphere at temperatures from 900xc2x0 C. to 1100xc2x0 C. The annealing time is preferably less than or equal to 16 hours.
Before producing the electrode, the active material obtained can be subjected to a surface treatment using an acid or basic solution.
Rapid cooling of the molten alloy can be achieved by casting it into the form of a plate in a cooled metal mold, for example, which produces a rate of cooling of the order of 10xc2x0 C.sxe2x88x921. The grinding can be carried out in a ball, roller or blade mill, or in a gas jet (jet milling).
The alloy can equally be obtained by a gaseous atomization process in which the molten alloy is ejected through a nozzle in the form of droplets that cool in contact with an inert gas atmosphere. The alloy is then obtained directly in the form of a powder.
The alloy can also be rapidly cooled using methods for rapid solidification of the melt by quenching on a wheel in an inert atmosphere (the melt spinning process and its derivatives). The wheel is generally made from a metal that is a good conductor of heat, such as a copper alloy. The melt spinning process consists of using gas pressure to spray the melt onto the wheel through a circular orifice or an elongate slot (in which case the method is referred to as planar flow casting). The melt overflow process, also known as strip casting, consists of pouring the melt into an intermediate tank from which it overflows into contact with the wheel. These processes form strips or flakes (if the alloy is brittle) whose thickness is less than or equal to 100 xcexcm and provide cooling rates up to 106xc2x0 C.sxe2x88x921.
The invention also provides an electrode including an electrochemically active material in the form of the hydridable alloy previously described. The electrode includes a conductive support and a layer containing said active material and a binder. The conductive support can be a two-dimensional support such as a solid or perforated strip, an expanded metal, a grid or a woven material, or a three-dimensional support such as a foam or a felt. The support is covered with a layer containing the electrochemically active material, a binder and usually a conductive material, but also yttrium compounds. The active layer can also include small quantities of additives to facilitate shaping of the electrode, such as a thickener or a texture stabilizer. The electrode can also undergo a surface treatment using an acid or basic solution before or after it is placed in the storage cell.
The invention finally provides a secondary storage cell, in particular a nickel-metal hydride storage cell, comprising a negative electrode containing the hydridable alloy previously described, a positive electrode whose electrochemically active material is a hydroxide which contains nickel, a polymer separator and an alkaline aqueous electrolyte. The positive electrode can be of the sintered type or include a foam support. It can contain a hydroxide based on nickel partly substituted with Co and/or Zn, a cobalt compound in the form of a conductive coating and possibly an yttrium compound such as Y2O3. The two electrodes are separated by a polyamide or polyolefin separator, possibly treated with acrylic acid or a polysulfone.